Method and apparatus for synchronizing the movements of a plurality of individually driven power devices



y 9, 1969 'r. N. LJUNG 3,458,788 7 METHOD AND APPARATUS FOR SYNCHRONIZING THE MOVEMENTS OF A PLURALIIY OF INDIVIDUALLY DRIVEN POWER DEVICES Filed Sept. 15,;1966 5 Sheets-Sheet 1 I z 2 9 f I 22 g I if I |---1 ,n 11L. M,- l a o 223 0 1 Z'EZ fl 9 11".." 7 J/ /0 {a l7 /2 FIG] T. N. LJUNG 3,458,788 METHOD AND APPARATUS FOR SYNCHRONIZING THE MOVEMENTS OF A PLURALITY OF INDIVIDUALLY DRIVEN POWER DEVICES Filed Sept. 13. 1966 5 Sheets-Sheet 2 Ju!y.29,1969 v rQN.

urz'rnon AND APPARATUS FOR smcuaomzme THE MOVEMENTS OF A PLURALITY OF INDIVIDUALLY DRIVEN POWER DEVICES Filed Sept. 1a, 1966 LJUNG 5 3,458,788

5 Sheets-Sheet 3 July 29, 1969 T. N. LJUNG 3,458,788

METHOD AND APPARATUS FOR SYNCHRONIZING THE MOVEMENTS OF A PLURALITY OF INDIVIDUALLY DRIVEN rowan DEVICES Filed Sept. 13. 1966 5 Sheets-Sheet 4 July 29, 1969 T. N. LJUNG 3,453,788

METHOD AND APPARATUS FOR SYNCHRONIZING THE MQVEMENTS OF v A PLURALI'X'Y OF INDIVIDUALLY DRIVEN POWER DEVICES Filed Sept. 13. 1966 5 Sheets-Sheet 5 United States Patent 3,458,788 METHOD AND APPARATUS FOR SYNCHRONIZ- ING THE MOVEMENTS OF A PLURALITY 0F INDIVIDUALLY DRIVEN POWER DEVICES Torsten Nicholaus Ljung, St. Eriksgatan 103, Stockholm, Sweden Filed Sept. 13, 1966, Ser. No. 579,096 Int. Cl. H02p 15/50 US. Cl. 318-74 6 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for synchronizing the movements of a plurality of power devices individually driven at approximately equal speeds from a starting position over equal distances wherein the distance to be covered by each power device is divided into steps of equal length and a sensing device detects the arrival of any power devices at the end of a step before the others, whereupon the drive of the first arriving power devices is interrupted until the slower devices arrive and thereafter all the devices are again simultaneously started, the procedure being repeated until all the power devices have covered the required distance.

The invention relates to a method and an apparatus for synchronizing the movements of a plurality of power devices, such as lifting devices for lifting floor slabs or the like in the construction of buildings according to the lift-slab system.

In the construction of buildings by means of the wellknown so-called lift-slab system, it is necessary that all the lifting devices co-operating in lifting the floor slabs or other building elements should operate simultaneously and with equal speeds so as to ensure that these building elements are not subjected to excessive stresses. To this end, it has been suggested to couple the lifting devices mechanically with each other. Such a direct mechanical coupling is, however, expensive and difficult to adapt to the existing circumstances on the building site since the relative positions of the various lifting devices in the horizontal plane will vary from case to case. It has also been suggested to use hydraulic lifting devices and to couple these devices hydraulically. As, however, the load of the various lifting devices is not the same, the lifting device subjected to the smallest load will start its lifting movement first, then the lifting device subject to the next larger load and so forth so that the lifting devices do not move synchronously but rather in succession.

In the construction of buildings by means of the liftslab system it is, therefore, desirable that the various lifting devices for lifting the floor slabs or the like can be driven individually and independently of one another in order that their operation can be adapted to the load to which they are subjected and also so as to obtain complete freedom in positioning the lifting devices in accordance with the requirements of the building to be constructed. When, however, each lifting device is provided with its own drive means, it is necessary that means are provided for synchronizing the operation of the various lifting devices in order that they may carry out their lifting movements substantially simutaneously and with equal speeds.

It is accordingly an object of the invention to provide a method for synchronizing the movements of a plurality of power devices which are individually driven with approximately equal speeds to be displaced simultaneously from a starting position over equal distance.

Another object of the invention is to provide a simple, inexpensive and reliable apparatus for carrying out the above-mentioned method.

According to the invention the distance to be covered by each of the power devices is divided into steps of equal length, and each device which, on having been advanced one such step, has obtained a lead, exceeding a predetermined limit value, over the slowest moving device of said plurality of devices, is stopped so as to wait until, by the advance of said slowest device, its lead has been reduced to at least said limit value, whereupon it is again started automatically for movement through the next step, which steps are repeated until all of said power devices have covered said distance.

An apparatus for carrying out said method preferaby comprises an individual electric drive means for each of said power devices, a switching means for each of said electric drive means for connecting and disconnecting said drive means, and a plurality of feeler means, each responsive to the movement of one of said devices and actuated each time said device has advanced one step, said plurality of feeler means controlling the actuation of said respective switching means in such a way that at least the slowest moving power device is advanced substantially continuously through its successive steps whereas the electric drive means of each of the other power devices is temporarily disconnected in case the device, on having advanced one step, is more than a predetermined distance ahead of said slowest device, and is again automatically connected when said slowest device has reduced its lag to at least said predetermined distance.

Further objects and advantages of the invention will appear from the following description in conjunction wtih the accompanying drawings showing by way of example embodiments of the invention.

In the drawings:

FIGURE 1 is a schematic illustration, partly in section, of a hydraulic lifting device adapted to be used in the construction of a building in accordance with the lift-slab system;

FIGURE 2 is a schematic view of an apparatus for synchronizing the operations of a plurality of lifting devices as shown in FIGURE 1;

FIGURES 3 and 4 are graphical representations illustratings the operation of the synchronizing apparatus; and

FIGURE 5 is a schematic illustration of another embodiment of the synchronizing apparatus.

In FIGURE 1 a hydraulic jack is shown comprising an upright cylinder 1 and a piston member 2, slidably mounted for vertical up and down movement in the cylinder 1. At its lower end the cylinder has a part 3 fixedly mounted by means (not shown) on the upper end of a supporting column or post 4. The cylinder 1 has fixed thereto a lower cross-yoke 5 comprising two parallel U-beams mounted on both sides of the cylinder.

The upwardly extending piston member 2 has its upper end secured to a mounting block 6 which carries a unit 7 comprising an electric motor and a pump driven by this motor for delivering oil or another suitable liquid under pressure to the cylinder 1 in a known manner. The conduits connecting the pump with the cylinder and the adjustable metering valves controlling the flow of the oil to and from the cylinder during the upward and downward stroke, respectively, of the piston member 2 are not shown but may be of conventional design.

An upper cross-yoke 8 is fixedly secured to the block 6 which yoke 8 is of similar design as the yoke 5 and likewise comprises two parallel U-beams arranged on both sides of the block. The two yokes 5 and 8 are connected to each other by two helical draw-springs 9.

Two suspension rods 10 extend vertically through the two yokes 5 and 8 on opposite sides of the jack 1, 2 and the post 4. The lower ends of the rods 10 pass through bores 11 of a building element 12 such as a floor slab,

roof element or the like. The element has a further bore 13 through which the post 4 extends. The element 12 is supported by washers 14 and nuts 15 screwed on the threaded lower ends 16 of the suspension rods 10. It is to be understood, that the element 12 is supported in a similar way by further posts 4 and corresponding lifting apparatus similar to those shown in FIGURE 1 and arranged in proper and desired position on the building site.

The suspension rods 10 are each provided with a row of equally spaced annular ribs 17 having, as shown, an upwardly tapering side wall and a downwardly facing flat annular shoulder. Each of the yokes and 8 comprises for each of the rods passing therethrough a pair of claws 18 slidably mounted in the yoke transversely of the rods and pressed by springs 19 against the rod 10 passing therebetween. The claws 18 each have a rod engaging surface extending substantially through half a circle and provided with ribs having a profile complementary to that of the ribs 17 of the rods 10'. It will be clear that the claws 18 thus hold the suspension rods against downward movement with respect thereto but allow the rods to be moved upwardly relative to the yokes 5 and 8. During the latter movement the claws 18 are moved outwardly against the pressure of the springs 19 by the sloping side walls of the ribs 17 sliding along the complementary sloping side walls of the ribs until they snap back again behind the shoulders of these ribs. The building element 12 may thus be lifted by means of the pumping unit 7 delivering oil under pressure to the cylinder 1 so as to raise the piston 2 whereby the upper yoke 8 with its claw 18 pulls the rods 10 upwardly. At the end of the upward stroke of the piston 2 the pressure in the cylinder is relieved whereby the piston moves again downwardly under the weight of the yoke 8 and the unit 7 as also by the action of the springs 9, during which downward movement the rods 10 are held by the claws 18 of the lower yoke 5.

The lower yoke 5 carries a micro-switch 21 having a switch arm or feeler 22 extending to a point within reach of the ribs 17 of the right-hand rod 10 so as to be actuated each time the rod has been lifted over a distance corresponding to the height of the ribs. Instead of the micro-switch 21 actuated by the ribs of the rod it is also possible to mount a micro-switch inside the lower yoke 5 so as to be actuated each time one of the claws 18 of this yoke returns to its holding position after having been pressed outwardly by the ribs of the upwardly moving rod.

The construction of the lifting apparatus as shown in FIGURE 1 forms as such no part of the invention and, if desired, other lifting apparatus of a different known type may be used, for instance those comprising a screw jack. It is essential, however, that the lifting device, whatever its type, is provided with a feeler means, such as the micro-switch 21, which, as it were, divides the upward movement of the building element 12 into a great number of small steps of equal length and which is actuated each time the building element has advanced one such step.

FIGURE 2 shows schematically an embodiment of an apparatus according to the invention for synchronizing the movements of a number of lifting devices of FIGURE 1 co-operating in the raising of the building element 12. The apparatus comprises an electric motor 23 of accurately and stepless regulable speed driving a shaft 24. The shaft 24 carries a number of contact drums 25 each forming part of a switching device 26 for controlling the op eration of the pumping unit 7 of one of the lifting devices. Although only one switching device 26 has been shown in the drawing, it is to be understood that there is one such switching device and consequently one drum 25 on the shaft 24 for each of the lifting devices used in lifting the building element 12 of FIGURE 1.

Each drum 25 has a cylindrical surface of electrically conductive material which surface is, however, over part of its width covered by a strip 27 of insulating material which strip extends over the greater part of the circumference of the drum leaving a gap 28 between its two ends where the conductive surface of the drum is uncovered. The strips 27 of all the drums have the same length and the same angular position with respect to the shaft 24.

A fixed brush 29 engages the surface of the drum 25 to the side of the strip 27 which brush is connected by a line 30 to one of the terminals of the electric motor of the unit 7, the other terminal being connected to one pole of an electric power source 31.

A C-shaped switching member 32 is mounted for sliding movement in two brackets 33. The member 32 carries two brushes 34 and 35 situated on diametrically opposite sides of the drum 25 and adapted to alternately engage the drum depending on the position of the member 32. The brush 35 is adapted to engage the conductive surface of the drum and is connected to the other pole of the electric power source 31. The brush 34, however, is situated opposite the insulating strip 27 on the drum surface.

The position of the member 32 is controlled by two electro-magnetic devices 36 and 37, respectively, each comprising a core secured to the member 32 and a coil surrounding this core. The devices 36 and 37 are arranged on opposite sides of the member 32 and act to displace this member in opposite directions. The coil of the electromagnetic device 36 is connected on the one side to the brush 34 and on the other side through a lead 38 to the brush 35. The coil of the electromagnetic device 37 lies in a circuit comprising an electric power source 39 and the micro-switch 21 actuated by the supension rod 10 of FIGURE 1.

In the position of the elements as shown, the brushes 29 and 35 engage the rotating drum 25, driven by the electric motor 23, so that the electric motor of the unit 7 is connected to the power source 31 through these brushes and the conductive drum surface. The pumping unit 7 thus operates to raise its suspension rods 10. As soon as the rod 10 has advanced One step the micro-switch 21 is closed for a short moment by a rib 17 of the rod 10 whereby the electro-magnetic device 37 is energized and shifts the switching member 32 to its other position in which the brush 35 lies clear of the drum 25 and the brush 34 engages the drum. If the drum at this moment has an angular position in which the brush 34 contacts the insulating strip 27, the circuit of the pumping unit 7 is broken and this unit stops. If, however, the brush 34 engages the drum 25 at the gap in the insulating strip, a circuit is closed from the power source 31 through the lead 38, the coil of the electro-magnetic device 36, the brush 34, the conductive drum surface, the brush 29, the electric motor of the pumping unit 7 and back to the power source. The device 36 is thus energized and returns the member 32 to its shown initial position so that the pumping unit is not disconnected and its operation is not interrupted. In the same manner, a pumping unit which has stopped, is started again if the brush 34 after first engaging the insulating strip 27, on further rotation of the drum 25 contacts the gap portion 28. It is to be noted that the micro-switch 21 is so arranged that immediately after its actuation by a rib 17 of the rod 10, its contact arm 22 (FIGURE 1) drops again behind this rib whereby the switch is again opened. Thus only a short current pulse is caused to flow through the coil of the electro-magnetic device 37. If necessary the electromagnetic device 37 may be constructed so that on being energized it actuates the slidable member 32 with a slide retardation.

The functioning of the synchronizing apparatus comprising a number of switching devices of FIGURE 1 and controlling an equal number of pumping units 7 may be best explained by reference to the diagrams of FIG- URES 3 and 4. In these diagrams the vertical axis represents the height of lift H and is divided into equal parts d which distance d represents the length of one lifting step as determined by the distance between two adjacent ribs 17 of a suspension rod 10. As above described, at the end of each such step, the switch 21 sends a current pulse through the circuit of the electro-magnetic device 37. The horizontal axis represents the time T and is likewise divided into parts of equal length 2, these time periods t representing the time which the slowest moving lifting device needs for advancing through one lifting ste d.

fn the diagrams of FIGURES 3 and 4 it is assumed that the various lifting devices move with slightly different speeds. It is further assumed that by known means (not shown) the motor 23 driving the shaft 24 has been so adjusted that it moves in synchronism with the slowest moving of the group of lifting devices, whereby, when this slowest moving lifting device has advanced one step and its switching member 32 is actuated, the drum 25 has reached an angular position in which the brush 34 contacts the gap 28 in the insulating strip 27 whereby, as above explained the operation of the pumping unit 7 is not interrupted. The progress of this slowest moving lifting device is thus represented in the diagrams of FIGURES 3 and 4 by the straight line A.

The diagram of FIGURE 3 illustrates the situation in which the rear edge 27' of the insulating strip 27, as seen in the direction of rotation of the shaft 24 indicated by the arrow 40 of FIGURE 2, has such an angular position at the repective moments of actuation of the switching members 32 of the various switching devices, that all those lifting devices which move slightly faster than the slowest moving lifting device, are stopped after advancing one step and are simultaneously started again at the end of the time interval t when said rearward strip edges 27' pass underneath the brushes 34 of the switching devices. The stepwise movements of two such lifting devices driven slightly faster than the slowest lifting device A are represented by the dotted lines B and C, respectively, in FIGURE 3. These lifting devices B and C after each step thus wait until the slowest lifting device has likewise advanced through this step before they move on. It will be seen that the maximum lead of the lifting devices B and C over the lifting device A curing at the end of each step forms only a fraction of the step length d which itself is already quite small. The various lifting devices thus advance substantially synchronously in their simultaneous lifting movement.

The diagram of FIGURE 4 illustrates the situation if the rear edge 27' of the several insulating drum strips 27 is advanced through an angle corresponding to a time period I with respect to the time t which the slowest moving lifting device needs for advancing one step d. The time length l of the diagram of FIGURE 4 thus represents the period in which during each rotation of the shaft 24, the brushes 34 of the various switching devices 26 lie opposite the conductive part 28 of the switching drums 25 and the time length indicated by i of FIGURE 4 represents the period in which, during each rotation of the shaft 24, the brushes 34 lie opposite the insulating strips 27 of the drums 25. In the diagram of FIGURE 4 a line A has been drawn parallel to the line A representing the movement of the slowest lifting device and situated a time length l to the left of the line A. It will be seen that a lifting device such as the one of which the movement is represented by the broken dotted line E is stopped at the end of each lifting step and is started again at the line A. A slower moving lifting device of which the advance is represented by the broken dotted line D is seen to advance uninterruptedly through its first three consecutive stepsd before the line D at th end of the third step arrives to the left of the line A and is stopped until it is started again, simultaneously with the lifting device represented by the line E, at the moment indicated by the line A. There is thus an area comprised between the lines A and A in which no interruption of the movements of the lifting devices occur.

By proper adjustment of the respective metering valves of the pumping units 7 and of the speed of the motor 23 and proper choice of the time length I it is possible to regulate the various lifting devices in such a way that their lines of movement fall for a substantial period of time within this area between the lines A and A so that they may operate uninterruptedly for a considerable number of successive steps a before their time lead over the slowest moving lifting device has become greater than the time length 1.

FIGURE 5 shows schematically another embodiment of the synchronizing apparatus of the invention in which corresponding parts are indicated by the same reference numerals as in FIGURE 2.

The apparatus again comprises an electric motor 23 of variable speed driving a shaft 24. The shaft 24 carries a number of switching discs 41, one for each pumping unit 7, which discs are for the greater part made of a non-magnetic material but each comprise a permanent magnetic part 42 in the form of a sector of a circle. These sector-shaped magnets 42 of the discs 41 all have the same angular position with respect to the shaft 24 and also have the same angular extent 'which is comparable to the angular extent of the conductive gap portion 28 of the drums 25 of FIGURE 2.

Each disc 41 controls the operation of a magnetically actuated switch element shown schematically at 43 and arranged closely adjacent of an end face of the disc. The contacts of the switch element 43 are normally closed but are opened each time the sector-shaped permanent magnetic part 42 of the rotating disc moves past the switch element. Magnetically actuated switches of this type are well-known in the art and therefore need not be described here in detail.

The switch element 43 is connected in an electric circuit which comprises a voltage source 39, the coil of a relay switch 44 and the micro-switch 21 actuated by the ribs 17 of the suspension .rod 1th on the upward movement of the latter. This circuit further comprises the coil of a holding relay switch 45, the contact of which is normally open and is connected in parallel with the micro-switch 21. The contact of the relay switch 44 is normally closed and is arranged in one of the leads connecting the electric motor of the pumping unit 7 with the electric power source '31.

The synchronizing apparatus of FIGURE 5 operates as follows. Assuming again that the electric motor 23 has been so regulated that its speed is synchronized with the lifting steps of the slowest moving lifting device, as above explained, the micro-switch 21 of this device is actuated at the end of each lifting step at the moment the permanent magnet 42 of the disc 41 is situated opposite the switch element 43. This switch element has, therefore, been opened by the magnet and as a consequence the relay switch 44 is not excited on the closing of the micro-switch 21 so that the running of the pumping unit 7 of this lifting device is not interrupted.

If one of the lifting devices has obtained such a lead over the slowest moving lifting device that at the moment of actuating of its micro-switch 21 the permanent magnet 42 of the relevant disc 41 has not yet reached its switch element 43, the latter is in its closed position so that the closing of the micro-switch causes the flow of a current pulse which excites the coils of the relay switches 44 and 45. Relay switch 44 opens its contact to interrupt the feeding circuit of the pumping unit 7 and holding relay switch 45 closes its contact bridging the micro-switch 21. As soon as on further rotation of the disc 41 the permanent magnet 42 moves opposite the switch element 43 so as to cause the latter to temporarily open its contacts, the exciting circuit of the relay switches 44 and 45 is interrupted, their contacts return to their normal positions as shown and the pumping unit 7 is started again.

It will be seen that the diagrams of FIGURES 3 and 4 as above discussed with reference to the embodiment of FIGURE 2, are also valid for the embodiment of FIGURE 5. The time length l of FIGURE 4 is in this case determined by the angular extensipn of the sector-shaped permanent magnetic disc part 42,. This angular extension can again be so chosen that the area between the lines A and A of FIGURE 4 has sufl'icient width to allow all the lifting devices, by proper adjustment of their metering valves, to advance uninterruptedly for a great number of successive lifting steps d.

While the invention has been illustrated and described with reference to specific embodiments thereof, it will be understood that other embodiments may be resorted to without departing from the invention. Thus the discs 41 may each be replaced by a cam disc and the magnetically actuated switch element 43 by a suitable switch actuated by this cam disc, the peripheral length of the cam of these cam discs determining in this case the time length l of the diagram of FIGURE 4.

I claim:

1. A method for synchronizing the movements of a plurality of power devices, said method comprising individually driving a plurality of power devices with approximately equal speeds for simultaneous displacement from a starting position over equal distances, dividing the distance to be covered by each of said power devices into steps of equal length, comparing the position of the first said device to reach the end of each step with the position of the slowest moving of said devices to determine if a lead of a predetermined limit value has been exceeded between the fastest and slowest moving devices, Stopping said first device until the slowest moving device has advanced to a position such that the lead has been reduced to said limit value, whereupon the device which has been stopped is again started for movement through the next step, the procedure being repeated through successive steps until all of the power devices have covered the required distance from start to finish.

2. The method of claim 1 in which said power devices are lifting devices for lifting building elements in the construction of buildings by means of the lift-slab system.

3. An apparatus for synchronizing the movements of a plurality of power devices which are individually driven with approximately equal speeds for simultaneous movement, said apparatus comprising an individual electric drive means for each of said power devices, a switching means for each of said electric drive means for energizing and deenergizing the latter, and a plurality of feeler means each cooperating with one of said power devices and each including a sensing element responding to the advance of its respective power device through successive steps of predetermined equal lengths, a means controlling the switching means for said power device and actuated by said sensing element on each of said successive steps of said latter device, and means coupling said respective switching means, as controlled by said respective feeler means, with each other such that they cause at least the slowest moving power device of said plurality of power devices to advance substantially continuously through its successive steps but cause the electric drive means of each of the other power devices to be temporarily disconnected in case such device at the end of one of its successive steps is more than a predetermined minimum distance ahead of said slowest moving device, said drive means being again automatically energized by its switching means as soon as said slowest moving device has reduced its lag to at least said predetermined minimum distance.

4. The apparatus of claim 3, in which the switching means for each of said electric drive means comprises a power supply circuit for said drive means and a rotatably mounted contact drum, said means coupling the respective switching means with each other comprising a common shaft on which he Cont t drums of all the switching means are mounted, means for driving said shaft with a variable speed, each drum having a cylindrical surface of electrically conductive material connected in said supply circuit of said drive means, saiddrum having a predetermined width, a part of which is covered by a layer of insulating material extending over part of the circumference of the drum, the switching means for each drive means further comprising a switching member carrying a first and second contact brush arranged on diametically opposite sides of said drum, said first brush lying opposite the electrically conductive surface part of the drum and being connected in said supply circuit and the second brush lying opposite said drum part carrying said insulating layer, said switching member being mounted for shifting movement substantially perpendicular to the axis of said drum whereby the two brushes may alternately engage the drum surface, a first elcctro-magnetic means for shifting said switching member to a first position in which said first brush engages the drum, said first electro-magnetic means comprising an exciting coil connected in series between said two brushes, and a second electro-magnetic means for shifting said switching member to a second position in which said second brush engages the drum, a second circuit containing and controlled by said control means of said feeler means, said second electro-magnetic means comprising an exciting coil connected in said second circuit, such that said control means causes a current pulse to flow through said latter coil each time said feeler means is actuated.

5. The apparatus of claim 3 in which each of said switching means comprises a control circuit, a first switch element connected in said circuit, a rotating switch actuating member adapted to actuate said switch element for a predetermined period of each revolution thereof, said means coupling the respective switching means with each other comprising a common shaft on which the switch actuating member of all of said switching means are mounted, and means for rotating said shaft with a constant but regulable speed, each electric drive means including a respective power supply circuit, the switching means for each drive means further comprising an electro-magnetic switch device having a switch contact in the feeding circuit of the electric drive means and an actuation coil connected in said control circuit, said control means of each of said feeler means comprising a second switch element arranged in said control circuit, whereby said electro-magnetic switch device is only actuated to disconnect said electric drive means if at the moment of actuation of said second feeler switch element said first switch element has not yet been actuated by said rotating switch actuating member, said electric drive means being again connected by said electro-magnetic switch device as soon as said rotating switch actuating member actuates said first switch element.

6. The apparatus of claim 5 in which each of said rotating switch actuating members is a disc mounted on said shaft and carrying a permanent magnet having substantially the shape of a sector of a circle, each of said first switch elements being arranged closely adjacent one of said discs and being a magnetically actuated switch which responds to the field of the permanent magnet each time this magnet on its rotation moves past said latter switch.

References Cited UNITED STATES PATENTS 2,262,141 11/1941 Holcomb 318-32l 2,443,657 6/1948 King 3l873 X 2,490,034 12/ 1949 Deakin 313-73 ORIS L. RADER, Primary Examiner A. G. COLLINS, Assistant Examiner 

